1. Field of the Invention
The present invention relates to a magnetic recording medium characterized by its direction of easy magnetization, a magnetic recording apparatus on which the recording medium is mounted, and a method and apparatus for manufacturing the magnetic recording medium.
2. Background Art
In order to obtain increased areal density in magnetic recording apparatus, a perpendicular magnetic recording technique has been proposed to replace the conventional longitudinal magnetic recording technique. In perpendicular magnetic recording, the direction of recorded magnetization created in a recording medium is perpendicular to a film plane. This technique has the advantage that the fine recorded magnetization is thermally stable. As the magnetic head for perpendicular magnetic recording, a dual element write-read head can be considered. The read head employs a magnetoresistive head that is conventionally used in longitudinal magnetic recording. The write head can employ a single pole type (“SPT”) head comprised of a main pole and an auxiliary pole. By employing a double-layer perpendicular recording medium comprising a recording layer and an underlayer of soft magnetic material formed on the back surface of-the recording layer, the SPT head can advantageously produce a stronger magnetic field than a ring-type head that is conventionally used in longitudinal magnetic recording. While there is a need to improve the perpendicular magnetic recording properties such as, for example, resolution and S/N, if the average grain size of the recording layer is reduced to improve S/N, or if the thickness of the recording layer is reduced to obtain a steeper head field gradient and thereby improve resolution, the thermal stability of the recording medium is adversely affected.
Another example of magnetic recording is oblique magnetic recording that is widely used in magnetic tape media. This is a technique in which the direction of recorded magnetization is inclined with respect to the direction normal to the film plane, and in which a ring head is used for recording. It can achieve a high SIN recording. In the case of magnetic tape media, it is well known that different recording characteristics are obtained when the direction in which recorded magnetization is inclined is reversed with respect to the direction of recording tracks. Specifically, when the direction of recorded magnetization is from the back surface of the tape medium toward the front surface thereof and the direction of recording tracks is from the upstream of the direction of transportation of the tape medium to the downstream thereof, better recording characteristics are obtained when the direction of recorded magnetization as projected on the tape medium plane is opposite that of the recording tracks. This is described in IEEE Transactions on Magnetics, MAG-19 (1983), pp. 1635-1637, for example. An attempt to apply this oblique recording technique to disc media is disclosed in JP Patent Publication (Kokai) No. 58-128023 A (1983). This publication teaches that when a ring head is used for recording, an improved reproduced output can be obtained when a projection of the longitudinal axis of the needle-like particles forming a magnetic recording medium on the substrate surface is substantially parallel to the circumference of the disc while, at the same time, the longitudinal axis is inclined with respect to a line normal to the substrate surface by 45° or more. Further, JP Patent Publication (Kokai) No. 9-212855 A (1997) describes the effect of inclining the axis of easy magnetization when a medium having an underlayer is recorded using an SPT head. According to this publication, an improved S/N can be obtained when a projection of the axis of easy magnetization on the magnetic recording medium is inclined by 90° with respect to the direction of tracks. None of these prior art documents, however, analyze the effect of the relationship between the transport direction of the magnetic recording medium and the direction of inclination of the axis of easy magnetization.
A conventional method of manufacturing magnetic recording media, such as longitudinal or perpendicular magnetic recording media, involves the deposition of a thin film of a magnetic material for forming a magnetic recording medium on a substrate by physical vapor deposition, such as vacuum deposition or sputtering. In a longitudinal magnetic recording medium, a recording layer is formed by epitaxial growth on the surface of an underlayer whose crystal orientation is controlled, and the crystals are oriented such that the axis of easy magnetization is parallel to the film surface. In the film plane, the individual crystal particles either have generally random axes of easy magnetization, or are provided with a weak anisotropy so that the magnetization is predisposed to be directed toward the circumference of the disc-shaped magnetic recording medium. The latter is realized by employing a substrate on the surface of which fine grooves called texture are mechanically formed. However, in order to improve recording density, the spacing between the magnetic head and the magnetic recording medium must be reduced. For example, JP Patent Publication (Kokai) Nos. 2001-14664 A and 2002-109729 A disclose techniques of providing circumferential anisotropy by employing a smooth substrate without texture and controlling the direction of the traveling particles during the formation of the thin film such that it is inclined away from a line normal to the substrate and toward the circumference.
A totally different manufacturing method is disclosed in JP Patent Publication (Kokai) No. 2000-48340 A, whereby magnetic nanoparticles are arranged in an ordered manner. This publication, however, does not describe the method of controlling the magnetic anisotropy of particles.
As mentioned above, the head for perpendicular magnetic recording comprises an SPT head comprised of a main pole and an auxiliary pole is used. A head structure has been proposed for improving the field gradient of the SPT head, in which a shield made of soft magnetic thin film is disposed near the main pole. Examples are disclosed in U.S. Pat. No. 4,656,546 and IEEE Transactions on Magnetics, Vol. 38 (2002), pp. 163-168, pp. 1719-1724, and pp. 2216-2218. The heads described in these publications are for perpendicular magnetic recording and are not for media with an inclined axis of easy magnetization. These prior art publications mention that the head of this type can provide a steeper field gradient than the conventional SPT head. However, they either make little mention about the reduction of magnetic field strength, or state that the reversal of magnetization of the perpendicular magnetic recording medium would be easier due to the increase in the in-plane components of magnetic field, or that the magnetic field strength could be ensured by making the main pole thicker and reducing the head-media spacing.
In the prior art, there has not been proposed a method of improving resolution and S/N without adversely affecting the thermal stability in the perpendicular magnetic recording technique in which a medium having an underlayer is recorded using an SPT head. Further, in the art of recording a medium having an underlayer in which the axis of easy magnetization is inclined with respect to a line normal to the medium, there has been no analysis of the case when the transportation direction of the medium is nearly parallel to the direction of a projection of the direction of easy magnetization on the medium plane.
Furthermore, in the above-mentioned prior art, when a perpendicular magnetic recording medium having an underlayer is recorded using an SPT head with the structure in which a shield made of soft magnetic thin-film is disposed near the main pole, sufficient magnetic strength cannot be obtained. If a material with a large magnetic anisotropy is employed for the recording medium in order to allow magnetic recording to be carried out at high areal density without damaging the thermal stability, satisfactory recording cannot be carried out in the perpendicular recording technique.